Open coil heater having spaced end coils

ABSTRACT

An open coil electric heater is provided having a helical heater coil element. The coil has a predetermined pitch between successive turns. In forming a coil run for the heater, a constant pitch is maintained between all turns, particularly the turns of the end portions of the coil, where the coil makes a reversal in the coil design pattern. A method for forming an electrical coil for an open coil heater wherein the pitch between successive turns of the coil is maintained approximately constant regardless of the coil run pattern imposed by the heater design.

i451 June 17, 1975 M t e n a m d a H [54] OPEN COIL HEATER HAVING SPACED END COILS ma I t|L a s n n. mmah ammo m Wuxi. imeo WANK 5223 6777 9999 /lll/ 220l 5279 2I23 8.3.7.9. 3170 2497 2.6.6.7. 3333 [75] Inventors: Lawrance W. Wightman, St. Louis;

Howard C. Gebhart, Chesterfield; Nick Tichy, Afftori, all of Mo.

[73] Assignee; Emerson Electric co., st. Louis, M0. Primary EmminefeVOlOdymi/r Y- Mayewsky Attorney, Agent, or Firm-Polster and Polster Filed'. Mar. 4, i974 Appl. No.; 447,649

[57] ABSTRACT An open coil electric heater is provided having a helical heater coil element. The coil has a predetermined pitch between successive turns. ln forming a coil run for the heater, a constant pitch is maintained between all turns, particularly the turns of the end portions of the coil, where the coil makes a reversal in the coil def [56] References Cited sign pattern A method for orming an electrical coil UNITED STATES PATENTS for an open coil heater wherein the pitch between successive turns of the coil is maintained approximately constant regardless of the coil run pattern imposed by the heater design.

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PATENTEDJUN l 7 ms SHEET OPEN COIL HEATER HAVING SPACED END COILS BACKGROUND OF THE INVENTION This invention relates to heater elements and in particular to electrical heater coils for open coil electric heaters. While the invention is described with particular emphasis in respect to such heaters, those skilled in the art will recongize the wider applicability of this invention.

Conventionally, an open coil heater includes at least one coil of electrically resistive wire wound helically about a centerline axis and formed into some predetermined design. A serpentine design commonly is used. In that design, the coil has a plurality of parallel straight conductor run portions connected at alternate ends by semicircular end portions, the entire coil being electrically continuous. The distance bettween successive wire turns is an improtant consideration in heater de sign. That is. the distance between successive wire turns along the straight conductor run portion of the coil necessarily decreases along the inner radius of the end portions of the coil, as the end portions are formed by bending the coil 180. It has been conventional practice to increase the distance between successive wire turns in the straight conductor run portions of the heater coil in order to provide sufficient distance between the turns of the end portion.

Spacing between wire turns recently has assumed added significance. Thus, for example, coil resistance can be changed either by changing the wire size of the coil or by adding additional turns of wire within a given axial length of the coil. It is desirable, in manufacturing electrical products, to possess the capability of substituting various materials for one another, as costs of certain materials continually increase. In particular, substitution for the nichrome wire commonly utilized for heater coils is desirable. Such substitution usually occurs between materials having different resistivities. Consequently, wire size diameter must be altered in order to obtain equivalent performance after substitution. A number of low cost substitute materials for nichrom wire are available, but their substitution involves larger wire size diameters. However, the coil of the substitute material physically requires more space if equivalent electrical performance is provided. Prior to the invention disclosed hereinafter, straight substitution of one material for another was not feasible because the larger diameter of the substitute coil material could not be accommodated along the coil end portion without causing electrical malfunction. In many applications, the distance between turns in the straight conductor run portions can not be increased sufficiently to accommodate the larger diameter wire along the coil end portion without changing the physical dimensions of the open coil heater. Tooling costs for making such an overall change usually eliminates any cost reduction obtainable by material substitution.

Even when materail substitution is not an issue, however, the coil construction heretofore prevalent results in a heater design where the end portions, under normal conditions, operate at a higher temperature than the straight conductor run portion. That is, since the wire turns of the end portions always have been closer to one another than the wire turns of the straight conductor run portions, heat transfer along the end portions is not as efficient and the end portions tend to reach higher operating temperatures than the straight conductor run portions of the same heater coil. Consequently, heater output must be rated lower than expected in order to compensate for the higher operating temperatures of the end portions. Where output is not so limited, heater life affected adversely.

The method and apparatus of our invention eliminates these prior art deficiencies. The heater coil is designed so that the spacing between turns along the inner radius of the end portion is equal approximately to the spacing between turns along the straight conductor run portions of the coil. Considerably closer spacing between turns can be achieved throughout the coil length without the problems of short circuited coils prevalent in the prior art. Closer spacing means that various materials having different resistivities can be used in a heater design without changing the physical dimensions of the open coil heater structure. Even where substitution is not a consideration, however, the spaced apart turns of the end portions permit higher heater ratings without adversely affecting heater life.

One of the objects of this invention it to provide an improved open coil heater element design.

Another object of this invention is to provide open coil heater elements haveing increased heater output and longer life.

Still a further object of this invention is to provide a serpentine design heater coil having relatively small distances between adjacent turns of the coil.

Yet another object of this invention is to provide a method for manufacturing an open coil heater having spaced apart end coils.

Further objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.

SUMMARY OF THE INVENTION In accordance with this invention, generally stated, an electrical coil is provided having successive turns of wire extending along a central axis. The distance between the successive turns is approximately equal along at least one side of the coil throughout the coil length, regardless of the silhouette pattern of the coil design.

In the preferred embodiment, the coil is an electrical heater coil for an open coil electric heater. The coil is a serpentine design having a plurality of coil end portions connecting a succession of parallel straight conductor run portions at alternate ends of the parallel conductor run portions. The turn spacing along the inner radius ofthe coil end portion is equal approximately to the spacing of the conductor run portion. A method for forming a heater coil is disclosed wherein the heater coil is stretched beyond the elastic limit of the material forming the coil. The end portions of the coil thereafter are expanded to provide the desired spacing between the successive turns of the end portions.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. I is a view in perspective, partly broken away, of an open coil heater utilizing a heater coil constructed in accordance with the principles of this invention;

FIG. 2 is a plan view ofa prior art heater coil having a large pitch straight coil run;

FIG. 3 is a plan view of a prior art heating coil having a small pitch straight coil run;

FIG. 4 is a plan view of coil of this invention;

FIG. 5 is a diagrammatic plan view ofone illustrative device for constructing the coil of FIG. 4, in a first operating position;

FIG. 6 is a diagrammatic plan view of the device of FIG. 5. in a second operating position;

FIG. 7 is a plan view, partly broken away, ofa second illustrative device for constructing the coil in FIG. 4; and

FIG. S is a sectional view taken alone the lines 8 8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. l, reference numeral l indicates an open coil heater which utilizes a plurality of individual heater coils 2 constructed in accordance with the principles of this invention. The coils 2 are mounted within a fram structure, generally indicated by the numeral 3. The frame 3 includes a front panel 4, a plurality of side members 5 and a plurality of support members 6 interconnected to one another by any convenient method. A particularly low cost and convev nient method for interconnecting structural members of the type used in heater constructions is disclosed in the co-pending Wightman et al US. application. Ser. No. 4l5,008, filed Nov, l2, i973. The support mem bers 6 have a plurality of coil support means 7 attached to them. Particular details of the coii support means 7 may be found in the co-pending Wightman et al US. application, Ser. NoA 415.007, filed Nov. I2, l973. Reference may be made to these co-pending applications for heater I constructional details not forming a part of this invention.

The front panel 4 has a plurality of controls 8 opera tively connected to one another and particular ones of the heater coil 2 plurality. The controls 8 perform the conventional functions of adjusting output of the heater I according to some predetermined arrangement. Operational aspects ofthe heater I form no part of this invention and are not described in detail.

Each of the coils 2 is a helical design having a plurality of individual turns 9 concentrically wound about a coil axis I0. The coil 2 design utilized in the open coil heater 1 is generally serpentine, having a plurality of straight conductor run portions ll connected at alternate ends by continuous sernieircular end portions l2. The coil 2 may be any desired length, and any number ofthe coils 2 maybe disposed within the heater 1 to develop a particular heater l rating. Those skilled in the art will recognize the wide design variations permissible in heater Construction.

As indicated above, it is conventional to form heater Coils in the serpentine silhouette design shown in FIG. l. However. all prior art coils differ in important constructional details from the coil 2 of this invention. FIG. 2, 3 and 4 are a graphic illustration of prior art coils compared to the coil 2 of this invention. In FIG. 2, a coil 13 has a straight conductor run portion 14 and an end portion l5. Individual turns of the coil 13, along the straight run portion 14. have a relatively large spacing I6 between them. With prior art techniques, when the coil I3 is formed or shaped along a radius R into the end portion I5, the spacing between adjacent turns of the coil I3 necessarily becomes smaller than the straight run portion I4 spacing 16. The end portion spacing is identified as spacing 16u. Since the coil 13 conventionally is not insulated electrically, the spacing I6 was determined so that sufficient clearance between adjacent turns was protided by the spacing 16u.

When additional turns in a given heater coil 2 length are desired, as for example, when it is desirable to inv crease the resistance of a give length coil, the spacing between successive turns of the coil must be decreased. However, prior art techniques have prevented the attainment of truely small spacing between successive turns because a small space between successive turns in the straight conductor run portion necessarily meant physical interference between successive turns along the end portion of the coil. This situation is illustrated by a coil I7 in FIG. 3. The coil 17 has a straight conductor run portion 18 which has a spacing 19 between successive turns of the coil 17, and an end portion 20. Once spacing 19 is decreased sufficiently', the turns of the end portion 20 come into Contact with one another along the inner radius R of the coil 17. Since the coil 17 is not insulated, contact between the individual turns comprising the coil 17 causes the coil 17 to short, which condition is unacceptable in open coil heater operation.

As previously described, coil 2 ofthis invention, illustrated in FIG, 4, has the end portion 12 extending between two straight conductor run portions l1. The coil 2 is helical and formed about the axis 19. The straight conductor run portion 11 has a spacing 2l between adjacent turns ofthe coil which spacing is maintained ap proxiinately constant throughout both the straight conductor run portion 1I and the end portion 12 ofthe coil 2, along at least one point in each successive turn forming the coil. The spacing of the turns along the outer radius of the end portion l2 is allowed to vary as the nature of end portion l2 construction always will cause that spacing to be equal to or greater than the spacing 21.

Coil 2 manufacture is accomplished on a device diagrammatically illustrated in FIGS. 5 and 6. As there shown, an end 22 of the coil 2 is attached to a holding means 23. Holding means 23 may be any convenient, low cost device. A post or stud attached to a support structure 25 works well, for example. The coil 2 is threaded about a plurality of capstans 24. The capstans 24 are arranged in two spaced, offset rows 30 and 50 respectively', that traverse the structure 25. The number of capstans 24 used to form a particular coil 2 will vary, and depends in large measure on the overall length of the coil 2 and the number of straight conductor run portions 11 finally present in the coil 2 design. At least one of the rows of capstans 24 is movahly mounted to the support structure 25. Various ones of the capstans 24 rows may be fixed, movabie in one plane, movable in two planes, or be describable in one or more of these categories. In the embodiment of FIGS. 5 and 6, the Capstans 24 ofthe row 30 are movable vertically with respect to the structure 25, as viewed in these Figures. That is, the capstans 24 have a first position illustrated in FIG. 5. In that position, the straight conductor run portions 1l of the coil 2 are not disposed parallelwise. The row 30 of the capstans 24 are moved downwardly to align the straight conductor run portion l1. The movement also thends to stretch the coil 2 beyond its elastic limits, thereby setting the coil 2 in its serpentine design, and permitting the coil 2 to maintain that design even after its removal from the capstans 24.

Support structure 25 is conventional and is not described in detail. Basically, its purpose is to provide a mounting means for the capstans 24 and related devices. Thus. the structure 25 may have grooves in it for defining capstans 24 limits of travel. Likewise, various cam structures may be attached to the structure 25 for controlling capstans 24 movement,

Capstans 24 are cone or frustum shaped devices having a narrow first end 26 and a broad base second end 27. The relative position of the ends 26 and 27, with respect to the base support structure 25 is not important to the overall concept of our invention. That is, certain ones of the capstans 24 may be mounted to the support structure 25 for movement normal to the structure 25. In the embodiment illustrated, all of the capstans 24 in the rows 30 and S0 are mounted for such movement. That movement either is inwardly or outwardly depending upon the relative location of the ends 26 and 27. While the capstan 24 movement is important in forming the coil 2, the relative direction of the movement is a design consideration that may vary in embodiments of our invention. In the embodiment illustrated, the capstans 24 move outwardly from the plane of the structure 25. A first position of the capstans 24 is shown in FIG. 5 while a second position of the capstans 24 is shown in FIG. 6. The FIG. 6 position diagrammatically illustrates capstan 24 position after the capstans have move normally to the plane of the structure 2S. The capstans 24 are powered by any convenient means. Conventional hydraulically operated cam actuators work well, for example. Movement of the capstans 24 outwardly from the base 25 expands the end portion l2 of the coil 2 about the taper of the capstans 24. That expansion increases the spacing between adjacent turns of the coil from the narrow spacing shown in FIG. 5 to the expanded spacing shown in FIG. 6. The capstans 24 may be provided with fin means 40 as an aid in holding the coil 2 in position during capstan 24 movement, if desired. Capstan 24 movement also sets the straight conductor run portions l1 in their final, approximately parallel relationship. Just as the vertical capstan movement leaves the straight conductor run portion in a permanent set condition, normal movement ofthe capstan 24 to the position illustrated in FIG. 6 stretches the end portion 12 of the coil 2 beyond the elastic limit of the end portion 12 so that the spacing between turns of the end portion l2 is retained after the coil 2 is removed from the capstan. Upon completion of end portion 12 expansion, the capstans 24 are retracted and the coil 2 may be removed for further processing. Spacing ofthe end portion l2 may be controlled by adjusting the de gree of normal travel of the capstans 24. While we find it convenient to adjust that travel so that the coil spacing remains relatively constant throughout the length of the coil 2, other embodiments of our invention may vary the spacing between turns.

From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects herein set forth, together with other advantages which are obvious and which are inherent to the structure.

Numerous variations, within the scope of the appended claims, will occur to those skilled in the art in light of the foregoing description and accompanying drawings. Thus, the silhouette of the coil 2 may be varied in other embodiments of this invention. Likewise. while a particular form ofa capstan 24 is described. the term capstan is intended to be generic for any device used to impart the spacing between turns ofthe end portions l2, and the particular form of these devices may be altered in other embodiments of this invention. For example, a second illustrative embodiment of this invention is shown in FIGS. 7 and 8, like reference numerals being utilized where appropriate. Again, the ends 22 ofthe coil 2 are attached to holding means 23 and the coil 2 is threaded about a plurality of capstans 24. The capstans 24 are arranged in two rows 71 and 72 and are movably mounted to a base plate 73 and a base plate 74 respectively. The plates 73 and 74 each are mounted to a pair of guide rods 75 and generally are similar to one another except that the plate 73 is movable along the guide rods 75 in response to a suitable force stimulus while the plate 74 is movably atY tached to the guide rods by suitable clamping means, not shown. For simplicity, only the base plate 73 is shown in the drawings and described in detail. Plate 73 includes a front part 85, a back part 86 and a pair of side bearing parts 87. The side parts 87 have openings in them defining two channels 88 which receive the guide rod pair 75 in a slip lit. A cover 89 may be provided, if desired, to improve apparatus appearance and to provide a convenient mounting surface for the holding means 23. A plurality of cylinders 79 are supported on the front part of the base plate 73. The cylinders 79 are operatively connected to individual ones ofthe capstans 24 through suitable bearing structures indi cated generally at 80. Capstans 24 are movable normally to the plane of the front part 85 of the base plate 73 and function to stretch the end turned portion of the coil 2 to form that portion as previously described. The guide rods 75 are attached to a vertical member 90 by a suitable number of guide members 76. The verical member 90 in turn is attached to an apparatus base 78 by any convenient method. A drive cylinder 77, indicated by phantom lines in FIG. 7, is operatively connected to the base plate 73 and operates to move the base plate 73 upwardly with respect to the base plate 74 to stretch the straight conductor run portions ofthe coil 2. The form of the apparatus shown in FIGS. 7 and 8 advantageous in that it is a simple construction and allows the relative distance of travel between the base plates 73 and 74 to be adjusted easily. That is, by loosening the clamping means securing the base plate 74 on the guide rods, the base plate 74 may be moved relative to the plate 73, thereby varying the initial starting position of the plates 73 and 74. Consequently, the distance traveled by the plate 73 in response to the force of the cylinder 77 may be increased or decreased relative to the plate 74 even though the cylinder 77 stroke remains unchanged. Various combinations of movable or nonmovable capstans may be utilized. Coils formed in accordance with the principles of this invention may find application in a variety of applications in addition to that described. These variations are merely illustrative.

Having thus described the invention, what is claimed and desired to be secured by letters patent is:

1. In an open coil heater assembly including a frame structure. means for mounting an electrical heater coil to said frame structure, and means for insulating an electrical heater coil from said frame structure, the improvement which comprises an electrical heater coil having spaced apart end turns, said electrical heater coil being formed from a plurality of spaced, continuous turns of resistance wire, said heater coil having a serpentine design including a plurality of straight conductor run portions approximately parallel to ane another` said straight conductor run portions being connected at alternate ends by a plurality of curved end portions, said curved end portions having an inner radius and an outer radius, the continuous wire turns of said straight conductor run portions and said curved end portions being stretched during the manufacture of said electrical heater coil in a direction approximately corresponding to the longitudinal axis of said straight conductor run portions so as to impart a first permanent set to said coil. and to provide a spacing between successive wire turns of said straight conductor run portions. the wire turns of said curved end portions further being stretched in a direction approximately corre sponding to a radius about which said curved end por tions are formed in said respective design so as to impart a second, additional permanent set to the curved end portions of said heater coil and provide a spacing between successive wire turns of said curved end portions. the spacing of the turns of said curved end portions, measured. along the inner radius of said curved end portions, being approximately equal to the spacing between the turns of said straight conductor run portions along at least one point of each turn of said coil throughout the length of each of said straight conductor run portions and said curved end portions forming said-heater coil, said first and said second permanent sets maintaining said heater coil in said serpentine design before placement in said electrical heater assembly, said mounting means holding said heater coil against movement after placement of said heater coil in 8 said electrical heater assembly.

2. ln an open coil electrical heater assembly including a frame structure, means for mounting an electrical heater coil to said frame structure, and means for insulating an electric heater coil from said frame structure, the improvement which comprises an electrical heater coil constructed from a coil of wire having a plurality of successive wire turns formed in it, said wire coil hav ing a serpentine design defined by a plurality of approx imately straight conductor run portions, and at least one curved portion, the wire turns of said straight conductor run portions and the wire turns of said curved portion being stretched in a tirst direction so as to impart a first permanent set to said coil and to provide a first spacing between successive wire turns of said straight conductor run portions, the wire turns of said curved portion further being stretched in a second direction during the manufacture of said coil so as to have a second permanent set imparted to them and to provide a second spacing between successive wire turns of said curved portion, said first and said second permanent sets permitting said coil to retain said serpentine design prior to placement in said electrical heater assembly, said first and said second permanent sets also providing a spacing increment between successive turns of said wire coil such that the minimum value of said second spacing between successive turns of said curved portion is at least approximately equal to the minimal value of said first spacing between the successive turns of said straight conductor run portions.

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[SEAL] F ORM IDO-1050 (1D-69) UNITED STATES PATENT OFFICE 3,890,148? Dated June 17, 1975 Inventor(s) Lawrance W. Wghtman and Howard C. Gebhar't line line

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It is certified that error appears in the above-'identified patent and that said Letters Patent are hereby corrected as shown below:

6, "life affected" should be "life is affected";

25 "haveing" should be "having" lO alone" should be "along'l 19 "fram" should be "frame" 6, "give" should be "given" 27, "axis 19" Should be "axis l0" 614, "thends" should be "tends";

29, "move" Should be "moved";

3M "ver'cal" should be "vertical" M3 "8 advantageous" should be "8 is advantageous";

l, "ane" should be "one".

Signed and Sealed this A ttes t:

RUTH C. MASON Attesring Ufficer C. MARSHALL DANN Commissioner of Parenrs and Trademarks 

1. In an open coil heater assembly including a frame structure, means for mounting an electrical heater coil to said frame structure, and means for insulating an electrical heater coil from said frame structure, the improvement which comprises an electrical heater coil having spaced apart end turns, said electrical heater coil being formed from a plurality of spaced, continuous turns of resistance wire, said heater coil having a serpentine design including a plurality of straight conductor run portions approximately parallel to ane another, said straight conductor run portions being connected at alternate ends by a plurality of curved end portions, said curved end portions having an inner radius and an outer radius, the continuous wire turns of said straight conductor run portions and said curved end portions being stretched during the manufacture of said electrical heater coil in a direction approximately corresponding to the longitudinal axis of said straight conductor run portions so as to impart a first permanent set to said coil, and to provide a spacing between successive wire turns of said straight conductor run portions, the wire turns of said curved end portions further being stretched in a direction approximately corresponding to a radius about which said curved end portions are formed in said respective design so as to impart a second, additional permanent set to the curved end portions of said heater coil and provide a spacing between successive wire turns of said curved end portions, the spacing of the turns of said curved end portions, measured. along the inner radius of said curved end portions, being approximately equal to the spacing between the turns of said straight conductor run portions along at least one point of each turn of said coil throughout the length of each of said straight conductor run portions and said curved end portions forming said heater coil, said first and said second permanent sets maintaining said heater coil in said serpentine design before placement in said electrical heater assembly, said mounting means holding said heater coil against movement after placement of said heater coil in said electrical heater assembly.
 2. In an open coil electrical heater assembly including a frame structure, means for mounting an electrical heater coil to said frame structure, and means for insulating an electric heater coil from said frame structure, the improvement which comprises an electrical heater coil constructed from a coil of wire having a plurality of successive wire turns formed in it, said wire coil having a serpentine design defined by a plurality of approximately straight conductor run portions, and at least one curved portion, the wire turns of said straight conductor run portions and the wire turns of said curved portion being stretched in a first direction so as to impart a first permanent set to said coil and to provide a first spacing between successive wire turns of said straight conductor run portions, the wire turns of said curved portion further being stretched in a second direction during the manufacture of said coil so as to have a second permanent set imparted to them and to provide a second spacing between successive wire turns of said curved portion, said first and said second permanent sets permitting said coil to retain said serpentine design prior to placement in said electrical heater assembly, said first and said second permanent sets also providing a spacing increment between successive turns of said wire coil such that the minimum value of said second spacing between successive turns of said curved portion is at least approximately equal to the minimal value of said first spacing between the successive turns of said straight conductor run portions. 